Liquid crystal display apparatuses for image display are widely used as the large-sized displays of flat-screen television systems and monitors, etc. The liquid crystal display apparatuses employ a backlight unit that applies illuminating light to a liquid crystal display panel from the back to enhance the luminance of the display screen.
There have been known the following two different types of backlight units for large-sized liquid crystal television systems. One type is known as a direct light type backlight unit in which a plurality of light sources, e.g. light-emitting diodes (LEDs) or cold-cathode fluorescent lamps (CCFLs), are arranged directly below a liquid crystal display panel as a display screen and directed toward the entire display screen to illuminate the liquid crystal display panel directly with light emitted from the light sources.
The other type is an edge-light type backlight unit in which a plurality of light source, e.g. LEDs or CCFLs, are disposed along a side edge of a lightguide plate having the same size as the display screen, and in which the wave guiding and emission of light are controlled to emit light from the entire main surface of the lightguide plate toward the display screen. It should be noted that a resin-molded product is used as the lightguide plate. There have also been proposed other types of backlight units. In one type, light sources are disposed under a lightguide plate to introduce light from the light sources into the lightguide plate to guide and emit the light from the entire main surface of the lightguide plate toward the display screen. In another type, a plurality of lightguide plates are arranged adjacent to each other to form an integrated light-exiting surface of the same size as the display screen (see Japanese Patent Application Publication Nos. 2006-134881, 2006-339650, 2007-134224, 2007-149657, 2007-329114, 2006-310045, and Hei 11-288611).
Recently, as liquid crystal television systems increase in size, the demand for reduction in weight and thickness of the television systems has been becoming stronger. In this regard, the conventional direct light type backlight unit has the following problem. Because a plurality of light source are disposed directly below the display screen, there are large differences in brightness on the display screen between spots directly below which light sources are present and spots having no light sources located directly below. Therefore, it is necessary in order to illuminate the entire display screen with uniform brightness without luminance unevenness to set a wide space between the display screen and the light sources. Consequently, the direct light type backlight unit suffers from an increased overall thickness.
With the conventional edge-light type backlight unit or the like that illuminates the display screen through a lightguide plate, it is necessary to ensure a lightguide surface of the same size as the display screen by disposing a large-sized lightguide plate of the same size as the display screen or laying a plurality of lightguide plates close to each other without any gap between adjacent lightguide plates. Therefore, the edge-light type backlight unit has the disadvantage that the lightguide plate weight increases. If injection molding is employed as a method of producing a large-sized lightguide plate, it is difficult to fill the resin material throughout the molding tool when producing a large-area and thin lightguide plate. This problem can be solved by using a large-sized molding machine and increasing the injection pressure to fill the resin material satisfactorily. This solution, however, is difficult to adopt because the use of a large-sized molding machine causes an increase in cost of equipment. In addition, it is difficult with the edge-light type backlight unit to locally illuminate the display screen by local dimming, for example, and hence difficult to reduce the power consumption. It is also necessary to take thermal measures because a plurality of light sources such as LEDs are densely disposed along an end edge of the lightguide plate.